1. Field of the Invention
The present invention relates to a multi-carrier CDMA transmission system, a transmitting apparatus and receiving apparatus used in this system, and a multi-carrier CDMA transmission method, and more particularly relates to a multi-carrier CDMA transmission system in which the weighting of respective sub-carriers is controlled in accordance with fluctuations in the propagation path in order to compensate for the orthogonality between spreading codes that is destroyed by the effects of the propagation path, a transmitting apparatus and receiving apparatus used in this system, and a corresponding CDMA transmission method.
2. Description of the Related Art
A multi-carrier CDMA (code division multiple access) wireless transmission system was proposed in 1993 in “Multi-carrier CDMA in indoor wireless radio networks” (N. Yee et al., 1993 IEEE Personal Indoor Mobile Radio Communications); since then, the application of this system to digital mobile communications systems has been studied.
This system is a system in which information symbols are duplicated in the direction of the frequency axis, and the respective symbols are multiplied by respective single chips of a spreading code, after which the spread signals are transmitted in parallel by means of a plurality of sub-carriers with different frequencies. As a result, in this system, since multiplication by a spreading code is performed in the direction of the frequency axis, code multiplexing of a plurality of information channels can be realized by multiplying an orthogonal spreading code. Furthermore, since the symbol rate is reduced and the symbol length is expanded by performing parallel transmission using a plurality of sub-carriers, it is possible to reduce so-called “multi-path interference”. This multi-path interference refers to a deterioration in characteristics that occurs as a result of transmitted signals arriving at the receiving part at different times via a plurality of different propagation paths (multi-path propagation paths) so that the signals interfere with each other, which is a problem in mobile communication environments.
Furthermore, in the case of the abovementioned multi-path propagation paths, frequency-selective fading occurs in which the fluctuation in the propagation path varies according to the frequency, so that the signal transmission quality varies according to the frequency. In the case of multi-carrier CDMA, however, since the signals are spread in the direction of the frequency axis, the signal transmission quality can be improved by the frequency diversity effect.
Meanwhile, as is shown in FIGS. 20(a) and 20(b), the received signals in a multi-carrier CDMA transmission system are multiplied in the frequency direction by the same code as the spreading code by which the signals were multiplied on the transmission side, so that the signals are subjected to despreading by combining the received signals of the respective sub-carriers across the spreading code period. As is shown in FIG. 20(a), in a case where the propagation path fluctuation of the respective sub-carriers is constant, the spreading codes that are totaled in the respective information channels are orthogonal to each other; accordingly, following despreading, the signals of the respective information channels can be completely restored. However, in the signals that are received after being propagated over the multi-path propagation paths, as is shown in FIG. 20(b), the respective sub-carriers are subjected to different amplitude and phase fluctuations, so that the orthogonality between the spreading codes is destroyed, and the signals of other information channels interfere and remain in the signals following despreading. As a result, the signal transmission characteristics deteriorate. Accordingly, a method has been proposed in which the interference between information channels is reduced by a combining process in which the received signals of the respective sub-carriers are multiplied by weight.
As is shown in FIG. 21(a), ORC is a method in which the reciprocals of the propagation path fluctuation values are used as weights. Since the propagation path fluctuations of the signals following multiplication by these weights is constant, the orthogonality between the spreading codes is completely preserved. However, in the case of sub-carriers in which the amplitude value of the propagation path fluctuation is small, noise included in the received signals is amplified as a result of multiplication by large weights, so that the signal power/noise power ratio (SNR) following despreading is small, thus causing a deterioration in the signal transmission characteristics. As is shown in FIG. 21(b), MRC is a method in which the propagation path fluctuation values are used as weights. Here, sub-carriers with a small SNR are multiplied by small weights, and sub-carriers with a large SNR are multiplied by large weights, so that the SNR following despreading is maximized; however, the orthogonality between the spreading codes is destroyed to a great extent, so that interference is generated between the information channels.
As is shown in FIG. 21(c), EGC is a method in which all of the sub-carriers are multiplied by equal weights regardless of the propagation path fluctuation. This method gives consideration both to the improvement of the SNR following despreading, and to preserving the orthogonality between the spreading codes; however, since the propagation path fluctuation and number of multiplexed information channels vary from instant to instant in a mobile communication environment, optimal values are not always obtained in such an environment.
As is shown in FIG. 21(d), MMSEC is a method using weights which are such that the mean square error between the signals following despreading and the signals that are actually transmitted is minimal, thus producing optimal values in which improvement of the SNR following despreading and compensation for the orthogonality between the spreading codes are given consideration in accordance with the propagation path conditions that fluctuate from instant to instant. Accordingly, it is indicated in Reference A that MMSEC is the method that shows the most favorable transmission characteristics. Furthermore, a method in which the optimal weighting is calculated using values such as the amplitude/phase fluctuation values of the individual sub-carriers, the noise power, the number of multiplexed information channels or the like is indicated in Reference A (described below) as the weighting control method used in MMSEC.